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1 The Periodic Law of Contents and Periodic Properties Objectives Explain the roles of Mendeleev and Moseley in the development of the periodic table. Describe the modern periodic table. Explain how the periodic law can be used to predict the physical and chemical properties of elements. Describe how the elements belonging to a group of the periodic table are interrelated in terms of atomic number. Mendeleev and Chemical Periodicity Mendeleev noticed that when the elements were arranged in order of increasing atomic mass, certain similarities in their chemical properties appeared at regular intervals. Repeating patterns are referred to as periodic. Mendeleev created a table in which elements with similar properties were grouped together a periodic table of the elements. Mendeleev and Chemical Periodicity, After Mendeleev placed all the known elements in his periodic table, several empty spaces were left. In 1871 Mendeleev predicted the existence and properties of elements that would fill three of the spaces. By 1886, all three of these elements had been discovered.

2 Moseley and the Periodic Law Periodicity of Atomic Numbers In 1911, the English scientist Henry Moseley discovered that the elements fit into patterns better when they were arranged according to atomic number, rather than atomic weight. The Periodic Law states that the physical and chemical properties of the elements are periodic functions of their atomic numbers. The Modern Periodic The Periodic is an arrangement of the elements in order of their atomic numbers so that elements with similar properties fall in the same column, or group. Objectives Explain the relationship between electrons in sublevels and the length of each period of the periodic table. Locate and name the four blocks of the periodic table. Explain the reasons for these names.

3 Objectives, Discuss the relationship between group configurations and group numbers. Describe the locations in the periodic table and the general properties of the alkali metals, the alkalineearth metals, the halogens, and the noble gases. Periods and Blocks of the Periodic Elements are arranged vertically in the periodic table in groups that share similar chemical properties. Elements are also organized horizontally in rows, or periods. The length of each period is determined by the number of electrons that can occupy the sublevels being filled in that period. The periodic table is divided into four blocks, the s, p, d, and f blocks. The name of each block is determined by the electron sublevel being filled in that block. The elements of Group 1 of the periodic table are known as the alkali metals. lithium, sodium, potassium, rubidium, cesium, and francium In their pure state, all of the alkali metals have a silvery appearance and are soft enough to cut with a knife. The elements of Group 2 of the periodic table are called the alkaline-earth metals. beryllium, magnesium, calcium, strontium, barium, and radium Group 2 metals are less reactive than the alkali metals, but are still too reactive to be found in nature in pure form. Hydrogen has an electron configuration of 1s 1, but despite the ns 1 configuration, it does not share the same properties as the elements of Group 1. Hydrogen is a unique element. Like the Group 2 elements, helium has an ns 2 group configuration. Yet it is part of Group 18. Because its highest occupied energy level is filled by two electrons, helium possesses special chemical stability.

4 Sample Problem A a. Without looking at the periodic table, identify the group, period, and block in which the element that has the electron configuration [Xe]6s 2 is located. Sample Problem A b. Without looking at the periodic table, write the electron configuration for the Group 1 element in the third period. Is this element likely to be more reactive or less reactive than the element described in (a)? Sample Problem B An element has the electron configuration [Kr]4d 5 5s 1. Without looking at the periodic table, identify the period, block, and group in which this element is located. Then, consult the periodic table to identify this element and the others in its group. The p-block elements consist of all the elements of Groups except helium. The p-block elements together with the s-block elements are called the main-group elements. The properties of elements of the p block vary greatly. At its right-hand end, the p block includes all of the nonmetals except hydrogen and helium. All six of the metalloids are also in the p block. At the left-hand side and bottom of the block, there are eight p-block metals.

5 The elements of Group 17 are known as the halogens. fluorine, chlorine, bromine, iodine, and astatine The halogens are the most reactive nonmetals. They react vigorously with most metals to form examples of the type of compound known as salts. The metalloids, or semiconducting elements, are located between nonmetals and metals in the p block. Sample Problem C Without looking at the periodic table, write the outer electron configuration for the Group 14 element in the second period. Then, name the element, and identify it as a metal, nonmetal, or metalloid. The metals of the p block are generally harder and denser than the s-block alkaline-earth metals, but softer and less dense than the d-block metals. In the periodic table, the f-block elements are wedged between Groups 3 and 4 in the sixth and seventh periods. Their position reflects the fact that they involve the filling of the 4f sublevel. The first row of the f block, the lanthanides, are shiny metals similar in reactivity to the Group 2 alkaline metals. The second row of the f block, the actinides, are between actinium and rutherfordium. The actinides are all radioactive. [Xe]4f 14 5d 9 6s 1 [Ne]3s 2 3p 5 [Ne]3s 2 3p 6 [Xe]4f 6 6s 2 Block Group Name M / NM / Md Reactivity

6 Objectives Define atomic and ionic radii, ionization energy, electron affinity, and electronegativity. Compare the periodic trends of atomic radii, ionization energy, and electronegativity, and state the reasons for these variations. Define valence electrons, and state how many are present in atoms of each main-group element. Compare the atomic radii, ionization energies, and electronegativities of the d-block elements with those of the main-group elements. Atomic Radii The boundaries of an atom are fuzzy, and an atom s radius can vary under different conditions. To compare different atomic radii, they must be measured under specified conditions. Atomic radius may be defined as one-half the distance between the nuclei of identical atoms that are bonded together. Atomic Radii, Atoms tend to be smaller the farther to the right they are found across a period. Periodic Trends of Radii The trend to smaller atoms across a period is caused by the increasing positive charge of the nucleus, which attracts electrons toward the nucleus. Atoms tend to be larger the farther down in a group they are found. The trend to larger atoms down a group is caused by the increasing size of the electron cloud around an atom as the number electron sublevels increases.

7 Atomic Radii, Sample Problem E Of the elements magnesium, Mg, chlorine, Cl, sodium, Na, and phosphorus, P, which has the largest atomic radius? Explain your answer in terms of trends of the periodic table. Ionization Energy An ion is an atom or group of bonded atoms that has a positive or negative charge. Sodium (Na), for example, easily loses an electron to form Na +. Any process that results in the formation of an ion is referred to as ionization. The energy required to remove one electron from a neutral atom of an element is the ionization energy, IE (or first ionization energy, IE 1 ). Ionization Energy, In general, ionization energies of the main-group elements increase across each period. This increase is caused by increasing nuclear charge. A higher charge more strongly attracts electrons in the same energy level. Among the main-group elements, ionization energies generally decrease down the groups. Ionization Energy, Sample Problem F Consider two main-group elements, A and B. Element A has a first ionization energy of 419 kj/mol. Element B has a first ionization energy of 1000 kj/mol. Decide if each element is more likely to be in the s block or p block. Which element is more likely to form a positive ion? Electrons removed from atoms of each succeeding element in a group are in higher energy levels, farther from the nucleus. The electrons are removed more easily.

8 Electron Affinity The energy change that occurs when an electron is acquired by a neutral atom is called the atom s electron affinity. Electron affinity generally increases across periods. Increasing nuclear charge along the same sublevel attracts electrons more strongly Electron affinity generally decreases down groups. The larger an atom s electron cloud is, the farther away its outer electrons are from its nucleus. Ionic Radii A positive ion is known as a cation. The formation of a cation by the loss of one or more electrons always leads to a decrease in atomic radius. The electron cloud becomes smaller. The remaining electrons are drawn closer to the nucleus by its unbalanced positive charge. A negative ion is known as an anion. The formation of an anion by the addition of one or more electrons always leads to an increase in atomic radius. Ionic Radii, Cationic and anionic radii decrease across a period. Periodic Trends of Radii The electron cloud shrinks due to the increasing nuclear charge acting on the electrons in the same main energy level. The outer electrons in both cations and anions are in higher energy levels as one reads down a group. There is a gradual increase of ionic radii down a group.

9 Valence Electrons Chemical compounds form because electrons are lost, gained, or shared between atoms. The electrons that interact in this manner are those in the highest energy levels. The electrons available to be lost, gained, or shared in the formation of chemical compounds are referred to as valence electrons. Valence electrons are often located in incompletely filled main-energy levels. example: the electron lost from the 3s sublevel of Na to form Na + is a valence electron. Electronegativity Valence electrons hold atoms together in chemical compounds. In many compounds, the negative charge of the valence electrons is concentrated closer to one atom than to another. Electronegativity is a measure of the ability of an atom in a chemical compound to attract electrons from another atom in the compound. Electronegativities tend to increase across periods, and decrease or remain about the same down a group. Electronegativity, Sample Problem G Of the elements gallium, Ga, bromine, Br, and calcium, Ca, which has the highest electronegativity? Explain your answer in terms of periodic trends.

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